1. Field of the Invention
The present invention relates to a method of mounting various types of electronic components on a printed circuit board (PCB) by means of solder, and in particular, to a method of mounting electronic components on a PCB using a reflow soldering method
2. Description of the Related Art
Conventionally, soldering has been used as a method of mounting electronic components on a PCB. Referring now to FIG. 1, a method of mounting electronic components using solder is next described. The example here explained pertains to double-sided reflow in which reflowing is performed to apply solder on both surfaces of a PCB.
Solder paste is first printed on lands that are provided on the PCB (Step 101). A summary of this printing process is shown in FIGS. 2A to 2C.
In this process, metal mask 13 is first arranged on PCB 15 as shown in FIG. 2A. Openings 12 of the same dimensions as lands 14 are formed in metal mask 13 in a pattern of arrangement that corresponds to the pattern of arrangement of lands 14 that are provided on the PCB. Metal mask 13 is then arranged such that openings 12 of metal mask 13 are each positioned over lands 14 of PCB 15.
Solder 11 is then rolled and moved across metal mask 13 by printing squeegee 10, whereby openings 12 in metal mask 13 are filled with solder 11 as shown in FIG. 2B. After filling openings 12 with solder 11, metal mask 13 is removed from PCB 15 as shown in FIG. 2C. Solder paste is thus printed on the lands.
Chip components, in particular, a surface-mount component such as a QFP (Quad Flat Package) or an SOP (Small Outline Package), are next mounted on the printed solder paste (Step 102). FIG. 3 shows a perspective view of PCB 15 and QFP chip 1 mounted on this PCB 15 at this time, and FIG. 4 shows a plan view of an area of this PCB 15 in which lands 14 are formed.
QFP chip 1 is provided with a plurality of leads 2 at a narrow pitch. Lands 14 of PCB 15 are provided at positions corresponding to leads 2. Solder 11 is applied to lands 14 in the above-described Step 101. Solder 11 is applied in regions having the same area as lands 14 as shown in FIG. 4 by printing using metal mask 13 in which openings 12 having the same dimensions as lands 14 have been formed as described hereinabove. QFP chip 1 is mounted such that each lead 2 rests upon each of regions on which this solder 11 has been applied.
The PCB on which the surface-mount component has been loaded is passed through a high-temperature reflow furnace, whereby the solder paste melts and the leads of the surface-mount component are soldered to the lands of the PCB (Step 103). The processes to this point complete the packaging of one side of the PCB. The PCB is next turned over such that the side on which components have not been packaged is directed upward (Step 104).
Solder paste is next printed (Step 105) and components are loaded (Step 106) as in Steps 101 and 102. Next, through-holes (T/H) components, which are components that are configured for mounting such that leads pass through T/H that have been formed in the PCB, are loaded (Step 107). The PCB is then passed through a reflow furnace as in Step 103 to solder the components (Step 108). Finally, components that cannot withstand the high temperature of the reflow furnace are manually soldered (Step 109), whereby the packaging of electronic components to the PCB is completed.
In the prior-art method of packaging electronic components that has been described above, tin-lead (Sn—Pb) solder is typically used as solder 11. However, this tin-lead solder contains lead, a toxic heavy metal, and this raises the problem of damage to the global environment that results from inappropriate disposal of used electronic equipment. As a solution to this problem, the use of lead-free solder has become preferable in recent years to prevent pollution of the environment.
Tin-silver (Ag) solder is well known as such a lead-free solder. Due to the stable characteristics of silver, the use of this tin-silver solder in place of tin-lead solder for packaging electronic components enables packaging of electronic components while maintaining the same level of reliability as in the prior art. However, the melting point of tin-silver solder is approximately 220° C., which is much higher than the approximately 183° C. melting point of tin-lead solder. For this reason, simply using tin-silver solder to carry out packaging that was performed using tin-lead solder is problematic, and further, simply using equipment without modification that was used in packaging with tin-lead solder to perform packaging with tin-silver solder is also problematic. In particular, when melting tin-silver solder having a melting point of 220° C. in a reflow furnace to perform soldering, the electronic components may in some cases reach temperatures in excess of 240° C. The temperature endurance of a typical electronic component is approximately 230° C. Thus, when tin-silver solder is used to package an electronic component, there is the problem that the temperature endurance of the various electronic components that are used must be raised.
Besides tin-silver solder, which has a high melting point, tin-zinc (Sn—Zn) solder is also known as a lead-free solder. The melting point of this tin-zinc solder is approximately 197° C., and this tin-zinc solder can therefore be used to package prior-art electronic components without modifying the equipment of the prior art.
When lead-free tin-zinc solder is used as solder 11, the solder bonding portions can be made sufficiently strong if an electronic component is used that has leads that are plated with a lead-free material.
If, on the other hand, an electronic component having leads that are plated with a solder containing lead is packaged on a PCB using lead-free tin-zinc solder, and in particular, if the electronic component to be packaged is an electronic component that is provided with leads at a narrow pitch such as a QFP or an SOP, the lead that is contained in the solder that is used in the plating is segregated at the interface of the lands in the reflow process. This segregation further progresses when the reflow process is again carried out when packaging on the opposite surface. The lead that has become segregated in this way forms a layer of low strength at the interface of the lands.
Due to this weak layer, when warping and twisting in the PCB 15 have occurred during subsequent fabrication processes and so on, the solder strength of the leads of electronic components is compromised, and the peeling away of the leads is further encouraged. Stress tends to concentrate at leads at the corners of an electronic component, and the solder strength in these areas is particularly reduced, resulting in a greater tendency for leads to peel away.
As described in the foregoing explanation, when a prior-art electronic component such as a QFP or SOP having leads that are provided at a narrow pitch and that are plated with solder containing lead (Pb) is packaged on a PCB using lead-free solder as a solder paste, it is difficult to obtain adequate bonding strength.